1. Field of the Invention
This invention relates to a mobile unit identifying system and a method of demodulating for the same.
2. Description of the Prior Art
Mobile unit identification systems are known. Such mobile unit identification systems have interrogators and responders. The interrogator identifies a responder by transmitting an interrogating signal and receiving a responding signal from the responder.
In one of such mobile unit identification systems, the interrogator transmits the interrogating signal through a microwave radio signal. A responder entering a service area of the interrogator receives the interrogating signal and effects a spectrum coding with a predetermined data. Then, the responder transmits the responding signal to the interrogator. The interrogator can recognize the responder because the interrogator has the predetermined data also and decode tile responding signal with the data.
FIG. 5 is a block diagram of such a prior art mobile unit identification system disclosed in Japanese patent application provisional publication No. 2-8770. In FIG. 5, an interrogator 401 continuously transmits an interrogating signal X obtained by modulating an oscillation signal from an oscillator 440 with predetermined data by the modulator 441 through an transmitting antenna 442. When a responder 404 enters a service area of the interrogator 401, the responder 404 detects the presence of the interrogating signal X received by a receiving antenna 443 and then, spread-spectrum-codes the interrogating signal X with an identification code stored in an identification code memory 444 provided thereto and transmits a spread-spectrum-coded interrogating signal X as a responding signal Y through a transmission antenna 446. The interrogator 401 receives the responding signal Y and demodulates the received responding signal Y with one of identification codes read from a memory table 447 provided therein and detects a reproduction condition of the demodulated response signal. When the reproduction condition is not sufficient, that is, synchronization between the interrogator and the responder is not obtained, the read identification code used in the responder 404 does not agree with the identification code read from the memory table 447. Then, the interrogator 401 reads another identification data from the memory table 447 and demodulates the received response signal. This operation is repeated until the reproduction condition of the received response signal is sufficient. When the reproduction condition is sufficient, the interrogator can identifies the responder and produces an output signal 450 obtained by demodulating the received response signal by the spread spectrum demodulator 449.
However, in the above-mentioned prior art, there is a problem that the spread spectrum modulator 445 is necessary in the responder 404 though the responder 404 is required to have a small size and a low power consumption. Moreover, in the interrogator 401, a circuit for identification between the interrogator 401 and the responder 404, so that sizes of the interrogator 401 was large.
A second prior art mobile unit identification system is disclosed in Japanese patent application provisional publication No. 1-182778.
FIG. 23 is an illustration showing a condition of the second prior art mobile unit identification system. FIG. 24. shows a timing chart of the second prior art mobile unit identification system.
The interrogator 302 transmits a communication start requesting signal R0 intermittently as show in FIG. 24. When a responder 301a enters the service area 303 of the interrogator 302, the responder 301a receives the communication start requesting signal R0 at a timing t1 as shown in FIG. 24 and generates and transmits a response signal A0 toward the interrogator 302 to start a communication with the interrogator 302. When the interrogator 302 finishes or interrupts the communication with the responder 301a, the interrogator 302 transmits a communication end requesting signal RN. When the responder 301a receives the communication end requesting signal RN at a timing ts, stops the communication operation for a predetermined interval .DELTA.t. It is assumed that another responder 301b which has not effected a communication entered the service area 303, the responder 301b generates the response signal A0 to start a communication operation with the interrogator 302. Therefore, both responders 301a and 301b do not perform the transmission operation at the same time except that these responders 301a and 301b enter the service area 303 at the same time.
In this second prior art mobile unit identification system, the interrogator 302 and the responders 301a and 301b can perform the transmission operation separately. However, in the identification system wherein a responder transmits a modulation signal by reflecting the interrogating signal from the interrogator, this communication operation is impossible because the responder 301a transmits the response signal A0 when the interrogator transmits no signal during transmitting the response signal A0.